Definition of heavy metal phytotoxicity in contaminated soils is important in adopting an official method for the assessment of contaminated soils. Dose-response relationships explain how the response changes as exposure dose changes. However, the results of tests or observations are commonly based on total concentration of heavy metals in environments, not the exposure dose that causes toxicity. The total concentration as a regulatory criterion to assess the toxicity of heavy metals in soils, but ignored the mobile and available distinction among different heavy metal sources. In the present study, the copper (Cu) phytotoxicity to barley root elongation was studied in 17 representative Chinese soils. Also, the EDTA-extractable Cu concentration was applied to evaluate Cu extractability, after which the relationships between EDTA-extractable Cu and total Cu concentration in different soils were established. The concentrations of EDTA-extractable Cu accounted for 89.6–91.2% of total added Cu in 17 Chinese soils. The multiple linear regressions using soil pH, organic carbon content (OC) and cation exchange capacity (CEC) were found to explain over 85% of the variance in Cu phytotoxicity thresholds based on EDTA-extractable concentration. The EDTA-extractable Cu dose-toxicity relationships incorporating soil properties were then derived by regression analysis. Soil properties (soil pH, OC, CEC and oxalate extractable Mn) and EDTA-extractable Cu concentration explained more than 90% of the variance in the toxicity response of barley root elongation. The new dose-toxicity relationships could be used to develop a reasonable remediation strategy for contaminated soils.